U.S. patent application number 10/765293 was filed with the patent office on 2004-10-28 for ultrasonic elastography with angular compounding.
Invention is credited to Varghese, Tomy, Zagzebski, James A..
Application Number | 20040215075 10/765293 |
Document ID | / |
Family ID | 34795447 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040215075 |
Kind Code |
A1 |
Zagzebski, James A. ; et
al. |
October 28, 2004 |
Ultrasonic elastography with angular compounding
Abstract
Ultrasonic strain measurements, which characterize the structure
of tissue, may be obtained by combining multiple echo signals
acquired at different compressions and at different angles. Such
angular compounding may improve the quality of the elastic signal
and provide at one time both an axial and lateral strain
measurement.
Inventors: |
Zagzebski, James A.;
(Madison, WI) ; Varghese, Tomy; (Madison,
WI) |
Correspondence
Address: |
QUARLES & BRADY LLP
411 E. WISCONSIN AVENUE, SUITE 2040
MILWAUKEE
WI
53202-4497
US
|
Family ID: |
34795447 |
Appl. No.: |
10/765293 |
Filed: |
January 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60464678 |
Apr 22, 2003 |
|
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|
Current U.S.
Class: |
600/442 ;
600/449 |
Current CPC
Class: |
G01S 7/52042 20130101;
A61B 8/4209 20130101; A61B 8/08 20130101; A61B 8/00 20130101; G01S
15/8995 20130101; A61B 8/485 20130101 |
Class at
Publication: |
600/442 ;
600/449 |
International
Class: |
A61B 008/00 |
Claims
We claim:
1. An ultrasonic elastography system comprising: an ultrasonic
transducer system adaptable to provide a set of echo signals from
at least two different angles and from a plurality of voxels in a
region of interest, the set of echo signals including first echo
signals taken with tissue of the region of interest in a first
compressive state and second echo signals taken with tissue of the
region of interest in a second compressive state; and a processor
combining at least portions of echo signals from different angles
to produce at least one compounded strain measurement for a voxel
in the region of interest.
2. The ultrasound elastography system of claim 1 wherein the
processor provides a strain extractor and a combiner operating
together to receive the set of echo signals and produce the
compounded strain measurement.
3. The ultrasound elastography system of claim 2 wherein the strain
extractor first extracts at least one strain measurement from
portions of at least two pairs of ultrasonic echo signals at
different angles and provides the strain measurements to the
combiner to combine the strain measurements to produce the
compounded strain measurement for the voxel.
4. The ultrasound elastography system of claim 2 wherein the
combiner first combines portions of at least two pairs of
ultrasonic echo signals from different angles to produce at least
two pairs of combined echo signals and then provides the combined
echo signals to the extractor which extracts the compounded strain
measurement for the voxel.
5. The ultrasound elastography system of claim 2 wherein the
processor receives an angle of the ultrasonic echo signals
underlying the strain measurement to convert the strain measurement
to an equivalent strain measurement along a predetermined strain
axis; and combines the converted strain measurements to produce the
compounded strain measurement.
6. The ultrasound elastography system of claim 5 wherein the
predetermined strain axis is an axis of compression of the region
of interest.
7. The ultrasound elastography system of claim 5 wherein the
predetermined strain axis is perpendicular to an axis of
compression of the region of interest.
8. The ultrasound elastography system of claim 5 wherein the
conversion of strain measurements to equivalent strain measurement
multiplies the strain measurements by a weighting function based on
a difference between an angle of the strain measurement and the
predetermined strain axis.
9. The ultrasound elastography system of claim 8 wherein the
predetermined strain axis is an axis of compression of the region
of interest and wherein the weighting function is
w(.theta.)=1/(cos.sup.2 .theta.-.nu. sin.sup.2 .theta.); where .nu.
is an estimate of Poisson's ratio of the tissue of the region of
interest; and .theta. is the angle between the angle of the strain
measurement and the predetermined strain axis.
10. The ultrasound elastography system of claim 8 wherein the
predetermined strain axis is an axis perpendicular to an axis of
compression of the region of interest and wherein the weighting
function is w(.theta.)=1/(sin.sup.2 .theta.-.nu. cos.sup.1
.theta.); where .nu. is an estimate of Poisson's ratio of the
tissue of the region of interest; and .theta. is the angle between
the angle of the strain measurement and the predetermined strain
axis.
11. The ultrasound elastography system of claim 2 wherein the angle
of compression changes with changes in the angle of the echo signal
and wherein the predetermined strain axis is an axis angled with
respect to the compression of the region of interest.
12. The ultrasound elastography system of claim 11 wherein the
weighting function is w(.theta.)=1/(cos.sup.2 .theta.-.nu.
sin.sup.2 .theta.); where .nu. is an estimate of Poisson's ratio of
the tissue of the region of interest; and .theta. is the angle
between the angle of the strain measurement and the predetermined
strain axis.
13. The ultrasound elastography system of claim 1 wherein one
compressive state is no compression.
14. The ultrasound elastography system of claim 1 wherein the
processor combines the set of echo signals at the different angles
to produce at least two compounded strain measurements of the voxel
along at least two strain axes.
15. The ultrasound elastography system of claim 14 wherein the two
strain axes are perpendicular.
16. The ultrasound elastography system of claim 14 wherein one of
the two strain axes are perpendicular to the compression.
17. The ultrasound elastography system of claim 1 wherein the
processor provides compounded strain measurements for multiple
voxels to provide a strain image output.
18. The ultrasound elastography system of claim 1 wherein the
processor provides an output of the group consisting of: Poison's
ratio of the tissue of the region of interest, and shear strain of
the tissue of the region of interest.
19. The ultrasound elastography system of claim 1 wherein the
processor receives a series of position signals from the ultrasonic
transducer system and including a combiner using the position
signals to match corresponding portions of the echo signals by
voxel to produce the compounded strain measurement.
20. The ultrasound elastography system of claim 1 wherein the
processor includes a correlator correlating values of the echo
signals over each voxel to determine a maximum correlation and
using the maximum correlation to match corresponding portions of
the echo signals produce the compounded strain measurement.
21. A method of ultrasonic elastography system comprising the steps
of: (a) collecting with an ultrasonic transducer, first echo
signals from at least two angles and from a plurality of voxels in
a region of interest in a first compressive state; (b) collecting
with the ultrasonic transducer second echo signals from the angles
and from the plurality of voxels in a region of interest in a
second compressive state; and (c) combining at least portions of
first and second echo signals from different angles to produce at
least one compounded strain measurement for a voxel in the region
of interest.
22. The method of claim 21 wherein step (c) includes an extraction
step extracting strain measurement and a combining step combining
information from echo signals of different angles to produce the
compounded strain measurement.
23. The method of claim 22 wherein the extraction step first
extracts at least one strain measurement from portions of at least
two pairs of ultrasonic echo signals at different angles and the
combining step combines the strain measurements to produce the
compounded strain measurement for the voxel.
24. The method of claim 21 wherein the combining step first
combines portions of at least two pairs of ultrasonic echo signals
from different angles to produce at least two pairs of combined
echo signals and the extracting step extracts strain from the
combined echo signals to produce the compounded strain measurement
for the voxel.
25. The method of claim 21 wherein the processor receives an angle
of the ultrasonic echo signals underlying the strain measurement to
convert the strain measurement to an equivalent strain measurement
along a predetermined strain axis; and combines the converted
strain measurements to produce the compounded strain
measurement.
26. The method of claim 25 wherein the predetermined strain axis is
an axis of compression of the region of interest.
27. The method of claim 25 wherein the predetermined strain axis is
perpendicular to an axis of compression of the region of
interest.
28. The method of claim 25 wherein the conversion of strain
measurements to equivalent strain measurement multiplies the strain
measurements by a weighting function based on a difference between
an angle of the strain measurement and the predetermined strain
axis.
29. The method of claim 28 wherein the predetermined strain axis is
an axis of compression of the region of interest and wherein the
weighting function is w(.theta.)=1/(cos.sup.2 .theta.-.nu.
sin.sup.2 .theta.); where .nu. is an estimate of Poisson's ratio of
the tissue of the region of interest; and .theta. is the angle
between the angle of the strain measurement and the predetermined
strain axis.
30. The method of claim 28 wherein the predetermined strain axis is
an axis perpendicular to an axis of compression of the region of
interest and wherein the weighting function is
w(.theta.)=1/(sin.sup.2 .theta.-.nu. cos.sup.2 .theta.); where .nu.
is an estimate of Poisson's ratio of the tissue of the region of
interest; and .theta. is the angle between the angle of the strain
measurement and the predetermined strain axis.
31. The method of claim 22 wherein the angle of compression changes
with changes in the angle of the echo signal and wherein the
predetermined strain axis is an axis angled with respect to the
compression of the region of interest.
32. The method of claim 21 wherein the weighting function is
w(.theta.)=(cos.sup.2 .theta.-.nu. sin.sup.2 .theta.); where .nu.P
is an estimate of Poisson's ratio of the tissue of the region of
interest; and .theta. is the angle between the angle of the strain
measurement and the predetermined strain axis.
33. The method of claim 21 wherein one compressive state is no
compression.
34. The method of claim 21 wherein the processor combines the set
of echo signals at the different angles to produce at least two
compounded strain measurements of the voxel along at least two
strain axes.
35. The method of claim 24 wherein the two strain axes are
perpendicular.
36. The method of claim 24 wherein one of the two strain axes are
perpendicular to the compression.
37. The method of claim 21 wherein step (c) produces compounded
strain measurements for multiple voxels to provide a strain image
output.
38. The method of claim 21 wherein step (c) provides an output of
the group consisting of: Poisson's ratio of the tissue of the
region of interest, and shear strain of the tissue of the region of
interest.
39. The method of claim 21 including the step of receiving a series
of position signals from the ultrasonic transducer and matching
corresponding portions of the echo signals by voxel to produce the
compounded strain measurement.
40. The method of claim 21 including the step of correlating values
of the echo signals over each voxel to determine a maximum
correlation, and using the maximum correlation to match
corresponding portions of the echo signals, produce the compounded
strain measurement.
41. The method of claim 21 wherein the echo signals are collected
at angles differing by no more than 5 degrees.
42. The method of claim 21 wherein the echo signals are collected
at angles differing by less than 1 degree.
43. The method of claim 21 wherein the echo signals are collected
at angles ranging over 180 degrees.
44. The method of claim 21 wherein the echo signals are collected
at angles ranging less than 90 degrees.
45. The method of claim 21 wherein the echo signals are collected
of voxels aligned within a single image plane.
46. The method of claim 21 wherein the echo signals are collected
of voxels distributed over a volume extending for multiple voxels
in two dimensions perpendicular to a direction of ultrasonic
propagation.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application 60/464,678 filed Apr. 22, 2003 hereby incorporated by
reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
BACKGROUND OF THE INVENTION
[0002] The present invention relates to ultrasonic medical imaging
and in particular to an improved apparatus and method for making
ultrasonic elastography images.
[0003] Elastography is a new imaging modality that reveals the
stiffness properties of tissue, for example, axial strain, lateral
strain, Poisson's Ratio, Young's Modulus, and other common strain
and strain-related measurements. The strain measurements may be
collected over an area and compiled as a two-dimensional array of
data, which may then be mapped to a gray or color scale to form a
strain "image". Analogously, strain measurements may be collected
over a volume displayed either three-dimensionally or as a series
of stacked two-dimensional images.
[0004] In quasi-static elastography, two images of the tissue may
be obtained by the ultrasound device in two different states of
compression, for example, no compression and a given positive
compression. The tissue may be compressed by an external agency
such as a probe or the like, or may be compressed by muscular
action or the movement of adjacent organs. Strain may be deduced
from these two images by computing gradients of the relative local
shifts or displacements in the images along the compression axis.
Quasi-static elastography is analogous to a physician's palpation
of tissue in which the physician determines stiffness by pressing
the tissue and detecting the amount the tissue yields under this
pressure.
[0005] In dynamic elastography, a low-frequency vibration is
applied to the tissue and the tissue vibrations are measured, for
example, using Doppler detection.
[0006] Ultrasonic elastographic images may have considerable image
noise arising from a number of sources. In quasi-static
elastography, one noise source is so-called "correlation artifacts"
which relate to the amount of overlap in the windows of correlation
as may be reduced improve resolution.
[0007] A number of techniques have been used to minimize these
image artifacts, including: global temporal stretching in which the
post-compression echo signal is stretched by a known compression
factor before estimating axial strain, wavelet de-noising where
wavelet techniques are used to smooth the displacement estimates in
the wavelet domain without losing edge information, and
multi-compression averaging in which multiple readings are taken
with smaller compression increments and averaged together.
[0008] Typically elastography produces a strain measurement only
along the axis of compression. However, lateral strain may be of
value both in deducing qualities like Poisson's Ratio and in
countering the effects of lateral motion in de-correlating the
axial displacement of the tissue. In addition, shear strain images
can also be obtained.
[0009] Two principal methods have been used to obtain lateral
strain. The first described in U.S. Pat. No. 6,270,459 interpolates
between successive axial rays or echo signals to provide a basis
for horizontal displacement measurement using a correlation
technique. A second technique makes the assumption that the tissue
being measured is incompressible and simply deduces lateral strain
from the axial strain.
SUMMARY OF THE INVENTION
[0010] The present invention provides a method and apparatus to
significantly decrease image artifacts in elastographic imaging by
using multiple angle acquisitions combined either before or after
strain is extracted. The inventors have determined that even a
relatively small angular difference between acquisitions provides
the necessary statistical independence of the measurements to
significantly reduce image noise. This angular compounding
concomitantly provides both axial and lateral strain
measurements.
[0011] Specifically, the present invention provides an ultrasonic
elastography system including an ultrasonic transducer system
adapted to provide a set of echo signals from at least two angles
from a plurality of voxels in a region of interest. The set of echo
signals includes first echo signals taken with the region of
interest in a first compressive state (e.g. no compression) and a
second echo signal taken with the tissue of the region of interest
in a second compressive state. A processor combines at least
portions of the set of echo signals from different angles to
produce at least one compounded strain measurement for voxel in the
region of interest.
[0012] Thus is it one object of the invention to provide a simple
method of decreasing the statistical fluctuation in elastographic
measurements to improve image quality and quantitative
accuracy.
[0013] The processor may use the angle of the ultrasonic echo
signal underlying the strain measurement to convert the strain
measurement to an equivalent strain measurement along a
predetermined strain axis and may combine the converted strain
measurements to produce the compounded strain measurement.
[0014] Thus is it another object of the invention to provide a
method of combining strain measurements made at different angles to
improve strain measurements made along a given angle by converting
the multiple strain measurements to a common strain axis before
combination.
[0015] The predetermined strain axis may be the axis of compression
or may be an axis lateral to that axis.
[0016] Thus it is another object of the invention to provide strain
measurements along arbitrary axes.
[0017] The angle of compression of the tissue may remain constant
for the different angles of echo signal. Alternatively, the angle
of compression may change with changes in the angle of the echo
signal.
[0018] Thus it is another object of the invention to flexibly
provide for single or multi-angle compressions to be used in
extracting additional information from the tissue.
[0019] These particular objects and advantages may apply to only
some embodiments falling within the claims and thus do not define
the scope of the invention.
BRIEF DESCRIPTION OF THE FIGURES
[0020] FIG. 1 is a simplified block diagram of an ultrasound
scanner suitable for use with the present invention;
[0021] FIG. 2 is cross-sectional view of tissue showing collection
of the ultrasonic echo signals obtained with the apparatus of FIG.
1 into echo signal sets having different ray angles;
[0022] FIG. 3 is an alternative embodiment of the ultrasonic
transducer system of FIG. 1 for multi-angled transducer and/or
compression plate movement;
[0023] FIG. 4 is a simplified representation of two echo signals
obtained in different compression states showing their relative
displacement;
[0024] FIG. 5 is a signal flow diagram showing processing of the
sets of FIG. 2 into angularly compounded elastographic images
showing axial and lateral strain;
[0025] FIG. 6 is schematic representation of an individual voxel of
tissue under compression showing angles used for the computation of
the weighting factors of FIG. 5;
[0026] FIG. 7 is a flow diagram similar to that of FIG. 5 showing
expansion of the technique to multiple compression angles;
[0027] FIG. 8 is a perspective representation of the acquisition of
scan data over a three-dimensional region of interest for
parametric imaging of a volume rather than a single plane; and
[0028] FIG. 9 is an ultrasonic transducer showing scanning using a
multi-element phased-array linear or curvilinear transducer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] Referring now to FIG. 1, an elastographic imaging system 10
suitable for use with the present invention may employ an
ultrasonic imaging machine 11 alone or in combination with an
external computer 30. Generally, the ultrasonic imaging machine 11
provides the necessary hardware and/or software to collect the
series of ultrasonic echo signals that can be further processed as
will be described by a processor 33 held within the ultrasonic
imaging machine 11 in the external computer 30.
[0030] An ultrasonic transducer 12, associated with ultrasonic
imaging machine 11, may transmit multiple ultrasonic beams 14a,
14b, and 14c (only three shown for clarity) toward a region of
interest 18 within a patient. Ultrasonic beam 14b, which typifies
the ultrasonic beams 14, produces echoes along different
measurement rays 16 acquired at different angles within each
ultrasonic beam 14. Different rays 16 will pass through different
volume elements (voxels) 26 within the region of interest 18 and
generally a given voxel 26 will have multiple rays 16 passing
through it at different angles from different ultrasonic beams
14.
[0031] The echoes along each of the rays 16 will be received by the
transducer 12 and converted to electrical echo signals 17 acquired
through the interface circuitry 22 of the ultrasonic imaging
machine 11. The interface circuitry 22 may provide amplification,
digitization, and other signal processing of the echo signals 17 as
is understood in the art. These digitized echo signals may then be
transmitted to a memory 35 for storage in subsequent processing by
processor 33 as will be described.
[0032] Ultimately, the processed echo signals 17 are used to
construct an image displayed on a graphic display 32. Input
commands from an operator may be received via a keyboard 34 and/or
a cursor control device 36 such as a mouse attached to the
processor 33 as is well understood in the art.
[0033] In one embodiment, the ultrasonic imaging machine 11 may be
an Aloka SSD 2000 scanner employing a phased array transducer with
a center frequency of 3.5 MHz with a 6 dB band of 640 percent.
Digitized echo signals from this ultrasonic imaging machine 11 may
be captured by a Gage Applied Science 12100 A/D board and provided
to the computer 30 for processing.
[0034] In one embodiment, the ultrasound imaging machine 11 may be
a scanner that provides an interface that enables the user to
utilize beam-steering on a linear array transducer. The ultrasound
beam can be steered at different angles as shown in FIG. 1. The raw
radio frequency data obtained at each beam-steered angle is stored
on the ultrasound system for either off-line or real-time
processing on the ultrasound system itself. The strain images or
elastograms will be displayed alongside the ultrasound B-mode
sonograms or superimposed on the ultrasound sonograms.
[0035] Generally, as shown in FIGS. 1 and 9, the ultrasound
transducer 12 may be a single element transducer manually steered
to acquire echo signals along different rays 16 of each ultrasonic
beam 14, or be a multi-element ultrasonic transducer 12
electronically steered by phased array operation to acquire echo
signals along different rays 16 of each ultrasonic beam 14. The
ultrasonic transducer 12 may be manually repositioned for each of
the different ultrasonic beams 14 or automatically moved by means
of a motorized stage 42 receiving position signals 41 from the
processor 33 via the interface circuitry 22. As shown in FIG. 2, a
multi-element linear or curvilinear array ultrasonic transducer 12
may be steered to collect a series of angulated beams with minimal
or no physical translation. Mechanical translation and angulation
may also be used with multi-element phased array transducers.
[0036] A position sensor 19 may be attached to the ultrasonic
transducer 12 to obtain position data 43 indicating the position
and orientation of the ultrasonic transducer 12, or in the case
where a motorized stage 42 is used, position data 43 may be
extracted from the position signals 41 as the motorized stage 42 is
moved.
[0037] Referring still to FIG. 1, during acquisition of the
ultrasonic echo signals 17, compression is applied to the patient
via a compression plate 44 oriented to compress the patient along a
transmission axis 46 generally aligned with the center rays 16 of
the ultrasonic beams 14. The compression plate 44 has a central
aperture allowing the ultrasonic transducer 12 to transmit
ultrasound therethrough. The compression plate 44 may be manually
operated or may be operated by means of a compression signal 47
obtained from the interface circuitry 22 and received by actuators
on the compression plate 44. Alternatively compression may be
applied by the ultrasonic transducer 12 itself or other forms of
compression may be used as are understood in the art.
Alternatively, compression can be applied using the transducer
itself, or with the attached compression plate in a freehand manner
or using a motorized hand-held compressor for controlled
compressions.
[0038] Referring now to FIG. 3 in an alternative embodiment of the
invention, the motorized stage 42 of FIG. 1 may be replaced with an
arcuate gantry arm 50 along which the ultrasonic transducer 12 may
travel in an arc about the patient in region of interest 18,
ideally over approximately 180 degrees. In this case ultrasonic
beams 14 orbit about a center rather than translate as shown in
FIG. 1. A compression plate 44 may be mounted with the ultrasonic
transducer 12 to apply compressions along the transmission axis 46
or may be fixed at one angle independent from the transmission axis
46. Again, position signals 41 may control the position of the
ultrasonic transducer 12 and compression signals may control the
compression plate 44 and position data 43 provided to the processor
33.
[0039] Referring now to FIG. 2, echo signals 17 along rays 16, from
either of the embodiments of FIG. 1 or 3, may be optionally
collected in projection sets 56 (only two projection sets 56a and
56b are shown for clarity) such that echo signals 17 in each
projection set 56 are along parallel rays 16. Each projection set
will obtain two different echo signals 17 and 17' (the latter
indicated by dotted lines), one corresponding each to two different
compression states of the compression plate 44.
[0040] The angle between measurement rays 16 of each projection set
56 may differ by as little as 0.75 degrees and still provide
sufficient independence of measurement to reduce the statistical
deviation in the measured strain. Nevertheless, higher degrees of
angular separation may also be used and in a preferred embodiment,
for example, forty-five different measurement sets 56 may be
obtained, each with one degree difference between them. Clearly
higher angular differences between the rays 16 or the measurement
sets 56 produce even greater statistical independence in the
measurements such as will help reduce noise. The invention
contemplates that angular separations of five degrees or greater
may also be practical over an angular ranges of up to 180 degrees.
An optimal angular separation exists that will be used when
possible under practical imaging conditions using beam-steering on
a linear array transducer.
[0041] Referring now to FIG. 4, generally within each projection
set 56, an echo signal 17 acquired during a first degree of
compression (e.g., no compression) will differ from an echo signal
17' acquired during the second degree of compression (e.g., a
predetermined positive compression) by a dilation of the echo
signal 17' along the horizontal axis or time axis with respect to
echo signal 17. In the example of FIG. 4, the echo signals 17 and
17' are raw ultrasound data, however, similar analysis can be
performed with B-mode or amplitude envelope data.
[0042] As is understood in the art, for each pair of echo signals
17 and 17', relative displacement between the echo signals 17 and
17' at each point along the length of echo signals 17 and 17' are
analyzed and through correlation or other techniques described
generally in U.S. patent application Ser. No. 2003/30171672 A1,
hereby incorporated by reference, to determine a displacement 60.
This analysis is confined to a window 58 determined according to
the desired resolution of strain measurement. This strain deduced
from displacement 60 will be axial strain, that is strain measured
along the transmission axis 46 of the echo signal 17. As is
generally understood in the art, each window 58 may be identified
with different volume elements 26 through which the particular rays
16 of echo signals 17 and 17' pass.
[0043] Referring now to FIG. 5 in a first step of the present
invention, echo signal sets 62 are acquired, each including echo
signals 17 and 17' at different compression amounts for one beam
14. Different echo signal sets 62 will hold data of different beams
14 taken at different locations and/or different angles by the
apparatus described with respect to FIGS. 1 or 4. The angle
information of each echo signal 17 is preserved within the echo
signal sets 62 as recorded by the processor 33 during acquisition
of this data.
[0044] In a next step, the echo signal sets 62 are optionally
provided to a rebinner 64 which separates them into parallel
projection sets 56 described above, each again having echo signals
17 and 17' associated with two sets of compression, however,
rebinned among echo signal sets 62 so that the echo signals of each
projection set 56 are along parallel rays. The particular
transmission axis 46 of the rays associated with each projection
set 56 is determined and used later. Rebinning is necessary only
when sector or phased array transducers are used.
[0045] A B-mode image (not shown) may be extracted from any or all
of either the projection sets 56 or echo signal sets 62 to produce
a reference image optionally to be projected together with the
elastographic image on display 32.
[0046] The projection sets 56 may then be provided to a strain
extracting block 66 which extracts strain information from the
projection sets 56 by comparing the echo signals 17 and 17' as
discussed above with respect to FIG. 4 to produce a series of
elastographic images 68 corresponding generally to each of the
projection sets 56. The elastographic images 68 will be composed of
pixels 70, each representing a strain vector 72 having a magnitude
proportional to the displacement 60 described above with respect to
FIG. 4, and an angle determined from the transmission axis 46
described above.
[0047] The elastographic images 68 on a pixel by pixel basis are
then weighted by weighting factors W and W,' to extract weighted
elastographic image pairs 74 comprised of an axial strain image 76
and a lateral strain image 76'. In this extraction, the pixels 70
of the axial image 76 provide strain extracted to a predetermined
axial direction 51 and the pixel 70 of the image 76' provides
extraction along a lateral direction 51' perpendicular to the axial
direction 51. It will be understood that axial direction 51 and
lateral direction 51' may be arbitrarily selected, however,
typically the axial direction 51 will be along the direction of
compression if there is one compression direction, and for the
embodiment of FIG. 3 where the direction of compression is changing
to a midrange of the gantry arm 50.
[0048] Referring now to FIG. 6, generally the weighting factor used
for this extraction will be a function of the angle .theta. between
the transmission axis 46 and the axial direction 51. The weighting
factor may be derived using a variety of methods, however, in the
preferred embodiment, it is deduced by assuming that the voxel 26
under compression along transmission axis 46 is an incompressible
material, in other words, a material having a Poisson Ratio of
approximately 0.5. In this case, a strain along transmission axis
46 may be multiplied by weight w(.theta.) to produce the equivalent
expected strain along transmission axis 46 according to the
following equation: 1 w ( ) = 1 ( cos 2 - v sin 2 ) ( 1 )
[0049] where v is the Poisson's ratio is equal to 0.495 for
incompressible media. Similarly, strain along transmission axis 46
may be multiplied by a slight variant of this weight W' to produce
strain along lateral direction 51', per the following equation: 2 w
( ) = 1 ( sin 2 - 1 / v cos 2 ) ( 2 )
[0050] Referring again to FIG. 5, the weighted strain vectors 72 of
the pixels 70 in each of the images 68 are thus resolved to axial
strain vectors 73 and lateral strain vectors 73' in elastographic
image pairs 74 allowing like-oriented strain vectors of each of the
elastographic image pairs 74 to be to be averaged per process block
80 to produce two images, an axial strain image 82 and a lateral
strain image 82'. This summing process reduces the noise in each of
the images 82 by an amount roughly equal to {square root}{square
root over (N)} where N is the number of images 68.
[0051] The axial strain image 82 and a lateral strain image 82' may
be mapped by means of mapper 84 to the graphic display 32. For
example, shades of gray may be used to map magnitudes of the axial
strain and different colors may be used to map magnitudes of the
lateral strain. Alternatively, the axial displacement and strain
image 82 and lateral displacement and strain image 82' may be
combined to produce shear strain images. Improved reconstruction of
Young's Modulus images can also be obtained with more accurate
normal and shear strain tensor information. A B-mode image may also
be displayed next to or superimposed over the lateral image or
switched with the lateral and axial images to provide a point of
reference for the particular strain values.
[0052] Referring now to FIG. 7, the weighting block 83 may provide
a more complex weighting system when the axis of compression may
move. In this embodiment, weights from weighting block 83 are
applied to the images 68 to produce a large number of compression
identified elastographic image pairs 74 each associated with a
different angle of compression axis. These may be combined per
process block 80 as before to produce an axial strain image 82 and
a lateral strain image 82' for each compression axis. Each of the
axial strain image 82 and lateral strain image 82' may, in its own
right, produce an image for a particular transmission axis 46 or
they may be combined by a combiner 90 to produce some directionless
quantity such as Poisson's Ratio or Young's Modulus from each of
these different measurements, such as made to be displayed on
graphic display 32.
[0053] Referring now to FIG. 8 for reasons of clarity, the
invention has been described with respect to voxels 26 aligned in a
single plane corresponding to a plane of the image produced on
graphic display 32. However, it will be understood the essential
principle of summing together the echo signals taken at different
angles to improve elastography measurements may occur by moving the
ultrasonic transducer 12 so as to collect multiple ultrasonic beams
that differ not only by their angle within the plane but also in
angles over a three-dimensional curved surface so as to produce a
volumetric image data that may be displayed one slice at a time or
rendered as a three-dimensional object.
[0054] It will be understood generally that the order of combining
the images and extracting the elastic strain information is
arbitrary and that in certain instances the images may be combined
first and then strain data extracted or strain data extracted and
then combined in an angularly sensitive way.
[0055] It is specifically intended that the present invention not
be limited to the embodiments and illustrations contained herein,
but include modified forms of those embodiments including portions
of the embodiments and combinations of elements of different
embodiments as come within the scope of the following claims.
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